By-elections (known as special elections in the United States) are used to fill elected offices that have become vacant between general elections.

In most cases these elections occur after the incumbent dies or resigns, but they also occur when the incumbent becomes ineligible to continue in office (because of a recall, ennoblement, criminal conviction, or failure to maintain a minimum attendance). Less commonly, these elections have been called when a constituency election is invalidated by voting irregularities.

In the United States, these contests have been called "special elections" because they do not always occur on Election Day like regular congressional elections. Despite their name, however, special elections to the U.S. House happen quite often. Furthermore, one published study shows that special elections are explained by the same factors as regular congressional elections.[1]

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The procedure for filling a vacant seat in the House of Commons of England was developed during the Reformation Parliament of the 16th century by Thomas Cromwell; previously a seat had remained empty upon the death of a member. Cromwell devised a new election that would be called by the king at a time of the king's choosing. This made it a simple matter to ensure the seat rewarded an ally of the crown.[2]

During the Parliaments of Charles II, which reached up to 18 years in length, by-elections were the primary means by which new members entered the House of Commons.[3]

In the United States, special elections are held when a seat in the House of Representatives, state legislature, or local legislature becomes vacant. At the federal level, the U.S. Constitution requires that vacancies in the House of Representatives be filled with a special election (unlike the Senate, where governors in most states have the power to appoint replacements).

When one seat in a proportional representation constituency becomes vacant, the consequences vary. For example, a by-election may be held to fill just the vacancy or all the seats in the constituency could be contested in the by-election.

Scotland and New Zealand still hold by-elections, despite having adopted the mixed-member proportional representation system, in which members are chosen by party lists. In both countries, by-elections where voters elect their preferred candidate are only used to fill a vacancy in a constituency seat. For example, the death of Donald Dewar resulted in a by-election for the constituency of Glasgow Anniesland. If a vacancy arises from the death or resignation of a party list member, the next unelected candidate on the party list is offered the seat. If that candidate has died or declines the seat, it is offered to subsequent candidates on the list until one accepts the seat. For example, on the resignation of Darren Hughes in March 2011, Louisa Wall was elected after all the five candidates above her on the New Zealand Labour Party's list declined the seat.[6] The Republic of Ireland holds by-elections despite electing members in multi-member constituencies by the single transferable vote.

keeping the seat vacant until the next general election. This usually occurs if a vacancy arises shortly before a planned general election (within six months in New Zealand).

nominating another candidate with the same affiliation as the former member – typically, in list systems, the next candidate on the party list.

For the Australian Senate (where each State forms a multi-seat constituency voting by single transferable vote), the State Parliament appoints a replacement; in 1977 a referendum amended the Constitution to require that the person appointed must belong to the same political party (if any) as the Senator originally elected to that seat. Those Australian states with an Upper House elected by PR-STV (NSW, Victoria and South Australia) copy the federal Senate model, except for Western Australia, which holds a recount of ballots, with sitting members retaining their seats.

Political scientists often caution against overinterpreting special election results, which non-experts often take as a bellwether. The evidence suggests that whilst they have some predictive power (particularly with margin of victory relative to the districts' normal performance, rather than the win-loss record); other indicators generally provide stronger evidence, and the often small data set of special elections per election cycle makes it worth extrapolating results cautiously.[9] Seats which have changed hands in "shock" by-elections also often revert to the former party in the next (or a subsequent) general election.

By-election upsets can have a psychological impact by creating a sense of momentum for one party or a sense of impending defeat for a government. Deborah Grey's 1989 by-election victory in Beaver River was seen as evidence that the newly formed Reform Party of Canada would be a serious political contender and that it posed a serious political threat for the ruling Progressive Conservatives. It also gave important momentum to the new party. Similarly, the upset 1960 by-election victory of Walter Pitman in Peterborough as a "New Party" candidate was a significant boost for the movement to replace the Co-operative Commonwealth Federation with an unnamed "New Party" which would be integrated with the labour movement. Pitman's candidacy in a riding in which the CCF was traditionally weak was seen as a test of this concept and his upset victory convinced the CCF and the labour movement to launch the New Democratic Party (NDP). Gilles Duceppe's 1990 upset landslide by-election victory in Laurier—Sainte-Marie with 66% of the vote on behalf of the newly formed Bloc Québécois was the first electoral test for what was initially a loose parliamentary formation created two months earlier after several Quebec MPs defected from the Progressive Conservative and Liberal parties to protest the failure of the Meech Lake Accord and provided the first indication that the party could be a serious force in the province of Quebec. On the strength of the by-election victory, the BQ went on to be officially formed as a party in 1991 and to win 54 seats in the 1993 federal election, enough to form the Official Opposition.

By-elections may occur singly, or in small bunches, especially if the authority responsible for calling them has discretion over the timing and can procrastinate. They are sometimes bunched to save money, as holding multiple by-elections is likely to cost more than holding a by-election to fill the vacancies all at once. In Canada, in 1978, 15 by-elections were held on a single date, restoring the House of Commons to 264 members. The media called it a "mini-election", a test of the Liberal government's popularity with a general election due in less than a year. The 15 districts stretched from Newfoundland to British Columbia, and produced some unexpected results, for example, an NDP candidate winning in Newfoundland for the first time. In September 1984 the Leader of the Greater London CouncilKen Livingstone and 3 other Labour councillors resigned and stood in simultaneous by-elections in an attempt to stage a mini-referendum on the Thatcher government's proposal to abolish the GLC. The effect of the manoeuver was blunted when the Conservative Party refused to stand candidates against them, and the following year the GLC was abolished.[10]

In Canada, General Andrew McNaughton was appointed to Cabinet as Minister of Defence on November 1, 1944 without having a seat in parliament after his predecessor resigned during the Conscription Crisis of 1944. A by-election was arranged in Grey North which the opposition Progressive Conservative party contested. The major campaign issue became the government's policy of "limited conscription" during World War II which McNaughton supported and which the Conservatives counterposed with a call for "full conscription". McNaughton was upset in the February 5, 1945 by-election. As a result, with confidence in his government undermined, Prime Minister William Lyon Mackenzie King called the 1945 federal election several weeks later when he had originally intended to wait until after the end of the war. McNaughton sought a seat in the federal election and resigned after he was again defeated.

In 1942, new Conservative Party leader Arthur Meighen sought to enter the Canadian House of Commons through a by-election in York South. His surprise defeat at the hand of Joseph Noseworthy of the Co-operative Commonwealth Federation ended his political career, and may also have been a factor in the Conservative Party's decision to move to the left and rebrand itself the Progressive Conservative Party under Meighen's replacement. Noseworthy's victory was also a significant breakthrough for the CCF giving it credibility as a national party where it has previously been seen as a Western Canadian regional protest party.

In 1965 the British Foreign Secretary Patrick Gordon Walker stood in the Leyton by-election for election to the UK Parliament, having been defeated in controversial circumstances in Smethwick at the previous year's general election. His appointment as a senior minister while not a member of either house of Parliament was against convention, and he therefore sought to regularise the position by standing in the first available by-election, which was at Leyton in January 1965. However a strong swing against Labour resulted in Gordon Walker's defeat: as a result, he resigned as Foreign Secretary.

In Canada, the most recent example of a cabinet minister appointed from outside of parliament having to resign after losing a by-election was in 1975 when Minister of Communications Pierre Juneau was appointed to Pierre Trudeau's Liberal cabinet directly from the private sector and tried to enter parliament through a by-election in Hochelaga. Juneau was upset by the Progressive Conservative candidate and resigned from cabinet ten days after his by-election defeat.

In Ireland, a by-election held in Dublin South-West provided a very surprising upset. The Sinn Féin candidate, Cathal King, was the favourite to take the seat. Sinn Féin had done extremely well in the area during that year's local elections. Sinn Féin captured high percentages of the first preference vote across the constituency. However, the Anti-Austerity Alliance candidate, Paul Murphy, was elected on the eighth count. Despite Murphy having received a lower first preference total than Cathal King, he outperformed the Sinn Féin candidate in attracting transfers. Murphy then took his seat in the 31stDáil. As a direct result of this defeat in the by-election, Sinn Féin hardened their stance against Irish Water and called for the complete abolition of water charges in Ireland.

The sole exception to this pattern was the Newry and Armagh by-election, where Seamus Mallon of the Irish nationalist and pro Anglo-Irish Agreement Social Democratic and Labour Party (SDLP) took the seat. The results of the 15 by-elections were cited by Unionists as a rejection of the Agreement by the Northern Irish electorate, but the action did not succeed in persuading the government of Margaret Thatcher to repeal the accord.